The objective of the project is to study in the dog if the microbial plaque is the major etiologic factor not only in overt gingivitis but also in progressive and destructive periodontitis. Twenty beagle dogs have been used. During a preparatory period of 8 weeks the animals were once weekly subjected to a careful prophylaxis and had their teeth brushed twice daily. At the end of this period the gingiva of all dogs were in excellent health. The dogs were then divided into two groups of ten; experimental and control. During a subsequent five year period the animals have been maintained in a state of adequate health and on a soft diet program which allows the formation of plaque. The teeth of the control dogs have twice daily been subjected to tooth brushing while the teeth of the experimental dogs have not been cleaned. Clinical examinations have been carried out regularly during the five years and biopsies have been sampled from different tooth regions. The results so far analysed have revealed that (1) it is possible in dogs to establish and maintain a non-inflammatory gingiva simply by eliminating calculus and subjecting the animals to meticulous plaque control (2) in dogs maintained plaque free over a five year period there were no clinical, roentogenologicad or histological signs of periodontal tissue destruction. On the other hand dogs which freely accumulated plaque (3) rapidly developed signs of gingivitis and eventually also clinical, radiographical and histopathological signs of periodontal tissues breakdown. In two out of ten dogs wth longstanding gingivitis and supragingival plaque and calculus formation, however, no signs of subgingival plaque and periodontal breakdown had occurred after four years of experiment. The reason for this inability of the two animals to develop subgingival deposits is at present obscure. Experiments are presently carried out in these two dogs attempting to convert the chronc gingival lesion into a destructive periodontitis by placing cotton floss ligatures around the neck of the crowns of certain premolars.